Samples or analytes for analysis in mass spectrometers are often ionized in an atmospheric environment, and the ions are then introduced into a vacuum chamber that contains the mass spectrometer. An atmospheric pressure ion source provides advantages in handling of samples, but the introduction of ions from the ion source into the vacuum chamber often requires a proper interface disposed between the ion source and the vacuum chamber. For instance, one common family of ionization techniques includes electrospray and its derivatives, such as nanospray, which provides a low flow. In all such techniques, a liquid sample, containing the desired analyte in a solvent, is caused to form a spray of charged and neutral droplets at the tip of an electrospray capillary. Once the spray is produced, the solvent begins to evaporate and is removed from the droplet, which is a process commonly referred to as desolvation. Accordingly, an important step in generating ions is to ensure proper desolvation. The electrospray source is usually coupled with some means of desolvation in an atmospheric pressure chamber, where desolvation can be enhanced by heat transfer to the droplets (radiation, convection) or/and counter-current flow of dry gas. The spray generally consists of a distribution of droplet sizes, and subsequently, the degree of desolvation will be different for each droplet size. Consequently, after desolvation, there is a size distribution for desolvated particles where there are large and heavy charged particles that may contaminate the aperture or conductance limit, thereby preventing the long-term stable operation of the mass analysis region, and/or introducing additional noise to the ion detector. This additional source of noise reduces the signal to nose ratio and thus, the sensitivity of the mass spectrometer.
The ions and the accompanying solvent molecules (neutrals) and charged particles, are transferred from the atmospheric pressure region to the low-pressure chamber of the mass spectrometer. Generally, the mass spectrometer operates less than 10−4 Torr and requires stages of skimmers or apertures to provide step-wise pressure reduction. Various methods for allowing the ions to enter while preventing the neutrals from passing into the mass spectrometer are well known. In U.S. Pat. No. 4,023,398, assigned to the assignee of the present invention and the contents incorporated here, as represented in FIG. 9, the mass spectrometer 32 is coupled to atmosphere by the interface region 15. A partition 3 with an entrance aperture 4 is provided to separate the atmospheric pressure from the first vacuum or lower pressure region 10 of the mass spectrometer 32 and a curtain gas 7 is supplied to prevent surrounding gases and neutrals 14 from entering the vacuum regions 10 & 11. The diameter of the entrance aperture 4 is chosen to limit the gas flow from the atmospheric region in order to balance the pumping capacity of the first and subsequent vacuum pumps 12 and 13 in the mass spectrometer region 32. A curtain plate 5 with an orifice 6 is located between the entrance aperture 4 and the spray 2. The purpose of the curtain plate 5 is to apply a flow of curtain gas 7 in the reverse direction of the spray 2. The curtain gas 7 has two functions: to divert the neutrals 14 from entering the aperture 4 and to desolvate the charge droplets so to release ions. In this method, charged particulates and heavy charged droplets that are not fully desolvated and remain as residual charged droplets may pass through the curtain gas flow and continue to travel downstream towards the entrance aperture 4.
U.S. Pat. Nos. 4,977,320, and 5,298,744, teach a method whereby a heated tube made from conductive or non-conductive material is used for delivering the ions/gas carrier/solvent flow into the low-pressure chamber. In such a configuration, the heated tube provides two distinct and separate functions; firstly, due to its significant resistance to gas flow, the tube configuration, namely its length and inner diameter, adjusts the gas load on the pumping system; secondly, the tube can be heated to effect desolvation and separation of ions from neutrals. With respect to the first function, this resistance can be provided, while keeping the tube length constant, to ensure laminar gas flow in the tube and the widest possible opening for inhaling the ion/gas carrier/solvent flow. Generally, a wider bore for the tube provides increased gas flow and hence more load on the pumping system; correspondingly, reducing the tube length provides less resistance to the gas flow, so as also to increase the gas flow and load on the pumping system. These two geometric parameters, bore and length, are obviously related and can be adjusted to provide the desired flow rate and flow resistance. The second function is provided by mounting a heater around the interface tube. The heat provided to the tube promotes desolvation of the ion flow, and also helps to reduce contamination of the surface of the tube, thereby reducing memory effects. An interface of this type is able to work best under strictly laminar flow conditions, limiting the variability of the tube length and tube bore. Additionally, the desolvation, which depends on temperature and residence time (inversely proportional to gas velocity through the tube) is related to the pumping requirements. As a rule, it is not possible to optimize all the desired parameters; in particular, it is desirable to minimize total mass flow to reduce pumping requirements, on the other hand to ensure best efficiency for transfer of ions into the mass spectrometer, a large diameter tube with high mass flow rates is desirable. In addition, the desolvation of ions is also affected by the diameter of the tube due to changes in residence time.
U.S. Pat. No. 5,304,798 attempts to satisfy both of these requirements by teaching a method whereby a chamber has a contoured passageway to provide both the desolvation function and the capillary restriction function. The opening of the passageway adjacent to atmospheric pressure has a wide and long bore while the opposite end of the passageway, ending within the vacuum chamber, has a smaller shorter bore. The electrospray source is place in front of the opening of the wide bore allowing the spray to pass directly into the passageway. The desolvation is performed within the wide bore region while the smaller bore provides the mass flow restriction. The entire spray is passed into the desolvation tube and any neutral or charged particulates or droplets not fully desolvated, will pass into the small bore. These particulates or droplets can accumulate in the small bore, which may cause blockage or they may pass through the small bore and enter the vacuum chamber leading to extensive contamination.
U.S. Pat. No. Re. 35,413 describes a desolvation tube and a skimmer arrangement where the exit of the desolvation tube is positioned off-axis to the skimmer. Offsetting the axis of the tube from the orifice of the skimmer is intended to allow the ions to flow through the orifice while the undesolvated droplets and particulates impinge upon the skimmer. This method does not take into consideration that the undesolvated droplets or charged particles, are not restricted to travel along the axis of the desolvation tube but follow a distribution across the bore. That is, this arrangement will only prevent undesolvated droplets and particulates traveling along the central axis from entering the orifice. An offset of the desolvation tube will not prevent droplets and charged particulates aligned with the offset location from entering the skimmer or to prevent an accumulation from building up around the orifice. In addition, it is expected that there would be a reduction of the ion current through the skimmer as a function of the offset.
In U.S. Pat. No. 5,756,994, a heated entrance chamber is provided, and is pumped separately. Ions entering this chamber through an entrance aperture are then sampled through an exit aperture that is located in the side of the chamber, off any line representing a linear trajectory from the entrance orifice. The intention of this off alignment is to prevent the neutral droplets or particles from entering the exit aperture. Pressure in this heated entrance chamber is maintained around 100 Torr. To the extent that this is understood, there is an independent pumping arrangement in the entrance chamber, and the shape of the chamber is not conducive to maintaining laminar flow, with the entrance aperture being much smaller than the cross-section of the main portion of the chamber itself. It is expected that significant loss of ion current to the walls of this chamber would occur in addition to obvious inefficiency of sampling from only one point of cylindrical flow through the exit aperture.
Another common type of atmospheric pressure ion sources uses the matrix-assisted laser desorption/ionization (MALDI) technique. In such a source, photon pulses from a laser strike a target and desorb ions that are to be measured in the mass spectrometer. The target material is composed of a low concentration of analyte molecules, which usually exhibit only moderate photon absorption per molecule, embedded in a solid or liquid matrix consisting of small, highly-absorbing species. The sudden influx of energy in the laser pulse is absorbed by the matrix molecules, causing them to vaporize and to produce a small supersonic jet of matrix molecules and ions in which the analyte molecules are entrained. During this ejection process, some of the energy absorbed by the matrix is transferred to the analyte molecules, thereby ionizing the analyte molecules. The plume of ions generated by each laser pulse contains not only the analyte ions but also charged particulates containing the matrix material, which may affect the performance of the mass spectrometer if not removed from the ion stream.